The Role of TRPM4 Gene Mutations in Causing Familial Progressive Cardiac Conduction Disease: A Further Contribution.

Progressive cardiac conduction disease (PCCD) is a relatively common condition in young and elderly populations, related to rare mutations in several genes, including SCN5A, SCN1B, LMNA and GJA5, TRPM4. Familial cases have also been reported. We describe a family with a large number of individuals necessitating pacemaker implantation, likely due to varying degrees of PCCD. The proband is a 47-year-old-patient, whose younger brother died at 25 years of unexplained sudden cardiac death. Three paternal uncles needed a pacemaker (PM) implantation between 40 and 65 years for unspecified causes. At the age of 42, he was implanted with a PM for two episodes of syncope and the presence of complete atrioventricular block (AVB). NGS analysis revealed the missense variation c. 2351G>A, p.Gly844Asp in the exon 17 of the TRPM4 gene. This gene encodes the TRPM4 channel, a calcium-activated nonselective cation channel of the transient receptor potential melastatin (TRPM) ion channel family. Variations in TRPM4 have been shown to cause an increase in cell surface current density, which results in a gain of gene function. Our report broadens and supports the causative role of TRPM4 gene mutations in PCCD. Genetic screening and identification of the causal mutation are critical for risk stratification and family counselling.

Intrinsic Electrical Remodeling Underlies Atrioventricular Block in Athletes.

[Figure: see text].

Analysis of electropharmacological effects of AVE0118 on the atria of chronic atrioventricular block dogs: characterization of anti-atrial fibrillatory action by atrial repolarization-delaying agent.

AVE0118, an inhibitor of IKur, Ito and IK,ACh, was in the drug pipeline for atrial fibrillation. To investigate the limitation of AVE0118 as an anti-atrial fibrillatory drug, we studied its electropharmacological effects particularly focusing on the anti-atrial fibrillatory action as reverse translational research. We adopted the chronic atrioventricular block beagle dogs (n = 4), having a pathophysiology of bradycardia-associated, volume overload-induced chronic heart failure, in which the atrial fibrillation was induced by 10 s of burst pacing on atrial septum. AVE0118 in doses of 0.24 and 1.2 mg/kg, i.v. over 10 min hardly altered electrophysiological variables. Meanwhile, AVE0118 in a dose of 6 mg/kg, i.v. over 10 min delayed the inter-atrial conduction in a frequency-dependent manner and prolonged the atrial effective refractory period in a reverse frequency-dependent manner, whereas it did not significantly alter the duration of atrial fibrillation or its cycle length. The increment of atrial effective refractory period was 3.3 times greater compared with that of ventricular one at a basic cycle length of 400 ms. Torsade de pointes was not induced during the experimental period. Thus, AVE0118 may possess a favorable cardiac safety pharmacological profile, but its weak anti-atrial fibrillatory effect would indicate the limitation of atrial repolarization-delaying agents for suppressing atrial fibrillation.

Cardiomyocyte Expression of ZO-1 Is Essential for Normal Atrioventricular Conduction but Does Not Alter Ventricular Function.

RATIONALE: ZO-1 (Zonula occludens-1), a plasma membrane-associated scaffolding protein regulates signal transduction, transcription, and cellular communication. Global deletion of ZO-1 in the mouse is lethal by embryonic day 11.5. The function of ZO-1 in cardiac myocytes (CM) is largely unknown. OBJECTIVE: To determine the function of CM ZO-1 in the intact heart, given its binding to other CM proteins that have been shown instrumental in normal cardiac conduction and function. METHODS AND RESULTS: We generated ZO-1 CM-specific knockout (KO) mice using α-Myosin Heavy Chain-nuclear Cre (ZO-1cKO) and investigated physiological and electrophysiological function by echocardiography, surface ECG and conscious telemetry, intracardiac electrograms and pacing, and optical mapping studies. ZO-1cKO mice were viable, had normal Mendelian ratios, and had a normal lifespan. Ventricular morphometry and function were not significantly different between the ZO-1cKO versus control (CTL) mice, basally in young or aged mice, or even when hearts were subjected to hemodynamic loading. Atrial mass was increased in ZO-1cKO. Electrophysiological and optical mapping studies indicated high-grade atrioventricular (A-V) block in ZO-1cKO comparing to CTL hearts. While ZO-1-associated proteins such as vinculin, connexin 43, N-cadherin, and α-catenin showed no significant change with the loss of ZO-1, Connexin-45 and Coxsackie-adenovirus (CAR) proteins were reduced in atria of ZO-1cKO. Further, with loss of ZO-1, ZO-2 protein was increased significantly in ventricular CM in a presumed compensatory manner but was still not detected in the AV nodal myocytes. Importantly, the expression of the sodium channel protein NaV1.5 was altered in AV nodal cells of the ZO-1cKO versus CTL. CONCLUSIONS: ZO-1 protein has a unique physiological role in cardiac nodal tissue. This is in alignment with its known interaction with CAR and Cx45, and a new function in regulating the expression of NaV1.5 in AV node. Uniquely, ZO-1 is dispensable for function of the working myocardium.

Cisd2 is essential to delaying cardiac aging and to maintaining heart functions.

CDGSH iron-sulfur domain-containing protein 2 (Cisd2) is pivotal to mitochondrial integrity and intracellular Ca2+ homeostasis. In the heart of Cisd2 knockout mice, Cisd2 deficiency causes intercalated disc defects and leads to degeneration of the mitochondria and sarcomeres, thereby impairing its electromechanical functioning. Furthermore, Cisd2 deficiency disrupts Ca2+ homeostasis via dysregulation of sarco/endoplasmic reticulum Ca2+-ATPase (Serca2a) activity, resulting in an increased level of basal cytosolic Ca2+ and mitochondrial Ca2+ overload in cardiomyocytes. Most strikingly, in Cisd2 transgenic mice, a persistently high level of Cisd2 is sufficient to delay cardiac aging and attenuate age-related structural defects and functional decline. In addition, it results in a younger cardiac transcriptome pattern during old age. Our findings indicate that Cisd2 plays an essential role in cardiac aging and in the heart's electromechanical functioning. They highlight Cisd2 as a novel drug target when developing therapies to delay cardiac aging and ameliorate age-related cardiac dysfunction.

LUF7244, an allosteric modulator/activator of Kv 11.1 channels, counteracts dofetilide-induced torsades de pointes arrhythmia in the chronic atrioventricular block dog model.

BACKGROUND AND PURPOSE: Kv 11.1 (hERG) channel blockade is an adverse effect of many drugs and lead compounds, associated with lethal cardiac arrhythmias. LUF7244 is a negative allosteric modulator/activator of Kv 11.1 channels that inhibits early afterdepolarizations in vitro. We tested LUF7244 for antiarrhythmic efficacy and potential proarrhythmia in a dog model. EXPERIMENTAL APPROACH: LUF7244 was tested in vitro for (a) increasing human IKv11.1 and canine IKr and (b) decreasing dofetilide-induced action potential lengthening and early afterdepolarizations in cardiomyocytes derived from human induced pluripotent stem cells and canine isolated ventricular cardiomyocytes. In vivo, LUF7244 was given intravenously to anaesthetized dogs in sinus rhythm or with chronic atrioventricular block. KEY RESULTS: LUF7244 (0.5-10 µM) concentration dependently increased IKv11.1 by inhibiting inactivation. In vitro, LUF7244 (10 µM) had no effects on IKIR2.1 , INav1.5 , ICa-L , and IKs , doubled IKr , shortened human and canine action potential duration by approximately 50%, and inhibited dofetilide-induced early afterdepolarizations. LUF7244 (2.5 mg·kg-1 ·15 min-1 ) in dogs with sinus rhythm was not proarrhythmic and shortened, non-significantly, repolarization parameters (QTc: -6.8%). In dogs with chronic atrioventricular block, LUF7244 prevented dofetilide-induced torsades de pointes arrhythmias in 5/7 animals without normalization of the QTc. Peak LUF7244 plasma levels were 1.75 ± 0.80 during sinus rhythm and 2.34 ± 1.57 µM after chronic atrioventricular block. CONCLUSIONS AND IMPLICATIONS: LUF7244 counteracted dofetilide-induced early afterdepolarizations in vitro and torsades de pointes in vivo. Allosteric modulators/activators of Kv 11.1 channels might neutralize adverse cardiac effects of existing drugs and newly developed compounds that display QTc lengthening.

Implantation of engineered conduction tissue in the rat heart.

Engineered conduction tissues (ECTs) are cardiac conduction tissues fabricated in vitro to allow for more precisely targeted in vivo transplantation therapy. The transplantation of ECTs may be ideal for the treatment of atrioventricular conduction block and could have a significant impact on the future application of biological pacemakers. However, there is little published information regarding the conduction function of ECTs in vivo. In the present study, ECTs were constructed by seeding cardiac progenitor cells (CPCs) into a collagen sponge and were then transplanted into animal hearts to determine whether they could act as an atrioventricular conduction pathway. The results demonstrated that the transplanted ECTs were adequately vascularized at the early stage of transplantation and could survive in the atrioventricular junction area of rats. A large number of myocardial tissue (≥29% of the new muscle fiber tissue formation area in the implanted ECTs) were observed by Masson's trichrome staining at 60 days post-transplantation. Positive staining for connexin-40, connexin-43, HCN2 and cTnT was exhibited during the period of 20 to 90 days post-transplantation. This result suggested that the transplanted ECTs formed gap junctions with the allogeneic myocardium and developed into cardiac conduction tissues with certain myocardial components. Electrocardiography (ECG) confirmed that there was a clear pre-excitation syndrome in the rats transplanted with ECTs during the period of 20 to 90 days post-transplantation. The recovery rate in the rats implanted with ECTs was 61.54% within 1 h following atrioventricular block, and the heart rhythm following recovery was close to normal. By contrast, the recovery rate was only 4.17% in the rats implanted with blank collagen sponges (BCSs), and none of the sham rats exhibited atrioventricular block recovery. In conclusion, ECTs can survive and mechanically integrate with the allogeneic myocardium following transplantation into rat hearts. An atrioventricular accessory pathway similar to Kent bundles could be established between the atria and ventricles of rats following implantation. It is suggested that ECTs may be a potential substitution therapy for atrioventricular conduction block.

Mutant KCNJ3 and KCNJ5 Potassium Channels as Novel Molecular Targets in Bradyarrhythmias and Atrial Fibrillation.

BACKGROUND: Bradyarrhythmia is a common clinical manifestation. Although the majority of cases are acquired, genetic analysis of families with bradyarrhythmia has identified a growing number of causative gene mutations. Because the only ultimate treatment for symptomatic bradyarrhythmia has been invasive surgical implantation of a pacemaker, the discovery of novel therapeutic molecular targets is necessary to improve prognosis and quality of life. METHODS: We investigated a family containing 7 individuals with autosomal dominant bradyarrhythmias of sinus node dysfunction, atrial fibrillation with slow ventricular response, and atrioventricular block. To identify the causative mutation, we conducted the family-based whole exome sequencing and genome-wide linkage analysis. We characterized the mutation-related mechanisms based on the pathophysiology in vitro. After generating a transgenic animal model to confirm the human phenotypes of bradyarrhythmia, we also evaluated the efficacy of a newly identified molecular-targeted compound to upregulate heart rate in bradyarrhythmias by using the animal model. RESULTS: We identified one heterozygous mutation, KCNJ3 c.247A>C, p.N83H, as a novel cause of hereditary bradyarrhythmias in this family. KCNJ3 encodes the inwardly rectifying potassium channel Kir3.1, which combines with Kir3.4 (encoded by KCNJ5) to form the acetylcholine-activated potassium channel ( IKACh channel) with specific expression in the atrium. An additional study using a genome cohort of 2185 patients with sporadic atrial fibrillation revealed another 5 rare mutations in KCNJ3 and KCNJ5, suggesting the relevance of both genes to these arrhythmias. Cellular electrophysiological studies revealed that the KCNJ3 p.N83H mutation caused a gain of IKACh channel function by increasing the basal current, even in the absence of m2 muscarinic receptor stimulation. We generated transgenic zebrafish expressing mutant human KCNJ3 in the atrium specifically. It is interesting to note that the selective IKACh channel blocker NIP-151 repressed the increased current and improved bradyarrhythmia phenotypes in the mutant zebrafish. CONCLUSIONS: The IKACh channel is associated with the pathophysiology of bradyarrhythmia and atrial fibrillation, and the mutant IKACh channel ( KCNJ3 p.N83H) can be effectively inhibited by NIP-151, a selective IKACh channel blocker. Thus, the IKACh channel might be considered to be a suitable pharmacological target for patients who have bradyarrhythmia with a gain-of-function mutation in the IKACh channel.

Disruption of RHOA-ROCK Signaling Results in Atrioventricular Block and Disturbed Development of the Putative Atrioventricular Node.

The RHOA-ROCK signaling pathway is involved in numerous developmental processes, including cell proliferation, differentiation and migration. RHOA is expressed in the atrioventricular node (AVN) and altered expression of RHOA results in atrioventricular (AV) conduction disorders in mice. The current study aims to detect functional AVN disorders after disturbing RHOA-ROCK signaling in chicken embryos. RHOA-ROCK signaling was inhibited chemically by using the Rho-kinase inhibitor compound Y-27632 in avian embryos (20 experimental and 29 control embryos). Morphological examination of control embryos show a myocardial sinus venosus to atrioventricular canal continuity, contributing to the transitional zone of the AVN. ROCK inhibited embryos revealed lateralization and diminished myocardial sinus venosus to atrioventricular canal continuity and at the severe end of the phenotype hypoplasia of the AVN region. Ex ovo micro-electrode recordings showed an AV conduction delay in all treated embryos as well as cases with first, second (Wenkebach and Mobitz type) and third-degree AV block which could be explained by the spectrum of severity of the morphological phenotype. Laser capture microdissection and subsequent qPCR of tissue collected from this region revealed disturbed expression of HCN1, ISL1, and SHOX2. We conclude that RHOA-ROCK signaling is essential for normal morphological development of the myocardial continuity between the sinus venosus and AVN, contributing to the transitional zone, and possibly the compact AVN region. Disturbing the RHOA-ROCK signaling pathway results in AV conduction disturbances including AV block. The RHOA-ROCK inhibition model can be used to further study the pathophysiology and therapeutic strategies for AV block. Anat Rec, 302:83-92, 2019. © 2018 Wiley Periodicals, Inc.

Critical Roles of Xirp Proteins in Cardiac Conduction and Their Rare Variants Identified in Sudden Unexplained Nocturnal Death Syndrome and Brugada Syndrome in Chinese Han Population.

BACKGROUND: Sudden unexplained nocturnal death syndrome (SUNDS) remains an autopsy negative entity with unclear etiology. Arrhythmia has been implicated in SUNDS. Mutations/deficiencies in intercalated disc components have been shown to cause arrhythmias. Human cardiomyopathy-associated 1 (XIRP1) and 3 (XIRP2) are intercalated disc-associated, Xin repeats-containing proteins. Mouse Xirp1 is necessary for the integrity of intercalated disc and for the surface expression of transient outward and delayed rectifier K+ channels, whereas mouse Xirp2 is required for Xirp1 intercalated disc localization. Thus, XIRP1 and XIRP2 may be potentially causal genes for SUNDS. METHODS AND RESULTS: We genetically screened XIRP genes in 134 sporadic SUNDS victims and 22 Brugada syndrome (BrS) cases in a Chinese Han population. We identified 16 rare variants (6 were in silico predicted as deleterious) in SUNDS victims, including a novel variant, XIRP2-E215K. There were also four rare variants (2 were in silico predicted as deleterious) detected in BrS cases, including a novel variant, XIRP2-L2718P. Interestingly, among these 20 variants, we detected 2 likely pathogenic variants: a nonsense variant (XIRP2-Q2875*) and a frameshift variant (XIRP2-T2238QfsX7). Analyzing available Xirp2 knockout mice, we further found that mouse hearts without Xirp2 exhibited prolonged PR and QT intervals, slow conduction velocity, atrioventricular conduction block, and an abnormal infranodal ventricular conduction system. Whole-cell patch-clamp detected altered ionic currents in Xirp2-/- cardiomyocytes, consistent with the observed association between Xirp2 and Nav1.5/Kv1.5 in co-immunoprecipitation. CONCLUSIONS: This is the first report identifying likely pathogenic XIRP rare variants in arrhythmogenic disorders such as SUNDS and Brugada syndrome, and showing critical roles of Xirp2 in cardiac conduction.

Transient receptor potential melastatin 4 cation channel in pediatric heart block.

OBJECTIVE: Progressive cardiac conduction disease (PCCD) is a common pediatric heart conduction disorder. It is an autosomal inheritance of rare mutations, which leads to familial cases of PCCD. In these cases, the His-Purkinje system's conductive capacity is progressively deranged, involving either right or left bundle branch block. Also, QRS complexes display widening is an important characteristic that culminates in complete AV block, syncope, and sudden death. Mutations in TRPM4 gene that encodes for transient receptor potential melastatin 4 have recently been reported to cause familial cases of PCCD and heart block. TRPM4 conducts a Ca2+-activated non-selective monovalent cationic current leading to a negative plasma membrane potential. TRPM4 channels let Na+ ion influx, causing membrane depolarization, whereas, at positive membrane potentials, TRPM4 channels repolarize the membrane by facilitating K+ ion efflux from the cell. TRPM4 protein contains many regulatory motifs that confer voltage dependence, ATP/ADP sensitivity, and Ca2+ responsiveness. Mutational studies revealed the significance of the two-calmodulin binding sites at the N-terminus of for Ca2+ dependent activation of this channel. Mutations that reduce deSUMOylation increase the steady-state levels of active TRPM4 channels on the membrane without alteration of its sensitivity to Ca2+ or ATP or its voltage dependence of activation. Increased TRPM4 function interferes with cardiac conduction and eventually contributes to heart block. Both gain and loss of function mutations of TRPM4 are implicated in the cardiac block. Currently, the major therapeutic management of cardiac block due to TRPM4 mutations is implantation of a pacemaker to reinstate normal current propagation through AV node.

Atrioventricular Conduction Delay Predicts Impaired Exercise Capacity in Patients with Heart Failure with Reduced Ejection Fraction.

BACKGROUND Atrioventricular conduction delay (AVCD) impairs left ventricular (LV) filling and consequently leads to a reduction of cardiac output. We hypothesized that in patients with severely depressed LV function and coexisting intraventricular conduction disturbances (IVCD), AVCD can affect exercise performance. Therefore, we evaluated the association of AVCD and exercise capacity in patients with heart failure (HFREF) and coexisting IVCD. MATERIAL AND METHODS We included patients with stable, chronic HFREF, LVEF <35%, sinus rhythm, and QRS ≥120 ms. PR interval and peak oxygen consumption (VO2 peak) were specifically investigated. Multiple regression analysis was used to adjust the association between PR interval and VO2 peak for possible confounders. RESULTS Most (57.5%) of the 40 included patients [20% female, aged 63±12, 47.5% of ischemic etiology (IHD)] were in NYHA class III. Mean PR interval was 196±38.1 ms. There were 26 (65%) patients with PR interval ≤200 ms and 14 (35%) with >200 ms. Groups were similar in clinical, laboratory, echocardiographic parameters, QRS morphology, and treatment regimens. VO2 peak was lower in patients with longer PR interval group as compared to shorter PR interval group (12.3±4.1 vs. 17.06±4.4, p=0.002). In the regression model, PR interval, female sex, and IHD remained important predictors of VO2 peak (partial=-0.50, p=0.003; rpartial=-0.48, p=0.005; rpartial=-0.44, p=0.01; R2=0.61). CONCLUSIONS Delayed AV conduction contributes to decreased exercise capacity in patients with HFREF and coexisting IVCD.

Short-term Variability of Repolarization Is Superior to Other Repolarization Parameters in the Evaluation of Diverse Antiarrhythmic Interventions in the Chronic Atrioventricular Block Dog.

Short-term variability (STV), to quantify beat-to-beat variability of repolarization, is a surrogate parameter that reliably identifies proarrhythmic risk in preclinical models. Examples include not only the use in the chronic atrioventricular block (CAVB) dog model whereby it was developed but also in vulnerable patients with heart failure or drug-induced long QT syndrome. In the CAVB dog model, STV can specifically distinguish between safe and unsafe drugs in proarrhythmic screening. Conversely, this dog model also offers the possibility to evaluate antiarrhythmic strategies in a setting of Torsades de Pointes (TdP) induction with a standard IKr inhibitor. The different antiarrhythmic interventions studied in suppression and prevention of drug-induced TdP in vivo in the CAVB dog model and in vitro in canine ventricular cardiomyocytes are described in this overview. We provide evidence that STV predicts the magnitude of antiarrhythmic effect against TdP better than other repolarization parameters in both suppression and prevention conditions. Moreover, suppression and prevention experiments revealed the same level of antiarrhythmic efficacy, whereas cellular experiments seem more sensitive in comparison with drug testing in vivo. Together, these observations suggest that STV could be used as a consistent indicator to rank efficacy of antiarrhythmic interventions in a number of conditions.

Induced Pluripotent Stem Cell-Derived Cardiomyocytes Provide In Vivo Biological Pacemaker Function.

BACKGROUND: Although multiple approaches have been used to create biological pacemakers in animal models, induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) have not been investigated for this purpose. We now report pacemaker function of iPSC-CMs in a canine model. METHODS AND RESULTS: Embryoid bodies were derived from human keratinocytes, their action potential characteristics determined, and their gene expression profiles and markers of differentiation identified. Atrioventricular blocked dogs were immunosuppressed, instrumented with VVI pacemakers, and injected subepicardially into the anterobasal left ventricle with 40 to 75 rhythmically contracting embryoid bodies (totaling 1.3-2×106 cells). ECG and 24-hour Holter monitoring were performed biweekly. After 4 to 13 weeks, epinephrine (1 µg kg-1 min-1) was infused, and the heart removed for histological or electrophysiological study. iPSC-CMs largely lost the markers of pluripotency, became positive for cardiac-specific markers. and manifested If-dependent automaticity. Epicardial pacing of the injection site identified matching beats arising from that site by week 1 after implantation. By week 4, 20% of beats were electronically paced, 60% to 80% of beats were matching, and mean and maximal biological pacemaker rates were 45 and 75 beats per minute. Maximum night and day rates of matching beats were 53±6.9 and 69±10.4 beats per minute, respectively, at 4 weeks. Epinephrine increased rate of matching beats from 35±4.3 to 65±4.0 beats per minute. Incubation of embryoid bodies with the vital dye, Dil, revealed the persistence of injected cells at the site of administration. CONCLUSIONS: iPSC-CMs can integrate into host myocardium and create a biological pacemaker. Although this is a promising development, rate and rhythm of the iPSC-CMs pacemakers remain to be optimized.

Prevalence and spectrum of NKX2.5 mutations in patients with congenital atrial septal defect and atrioventricular block.

Congenital atrial septal defect (ASD) and progressive atriventricular block (AVB) are the two most common phenotypes linked to NK2 homeobox 5 (NKX2.5) mutations in animals and humans. However, the prevalence and spectrum of NKX2.5 mutation in patients with ASD and AVB remain to be elucidated. In the present study, the coding exons and flanking introns of the NKX2.5 gene, which encodes a homeobox­containing transcription factor essential for development of the heart, were sequenced in a cohort of 62 unrelated patients with ASD and AVB, and subsequently in a mutation carrier's available family members. As controls, 300 unrelated, ethnically­matched healthy individuals were recruited, who were also genotyped for NKX2.5. The functional consequence of the mutant NKX2.5 was evaluated in contrast to its wild­type counterpart using a dual­luciferase reporter assay system. As a result, a novel heterozygous NKX2.5 mutation, p.Q181X, was identified in an index patient with ASD and AVB, with a prevalence of ~1.61%. Genetic analysis of the proband's pedigree revealed that the mutation co­segregated with ASD and AVB with complete penetrance. The nonsense mutation, which eliminated partial homeobox and the carboxyl terminus, was absent in the 600 control chromosomes. Functional evaluation showed that the NKX2.5 mutant had no transcriptional activity. Furthermore, the mutation disrupted the synergistic activation between NKX2.5 and GATA binding protein 4, another cardiac core transcription factor associated with ASD. The results of the present study expand the spectrum of NKX2.5 mutations linked to ASD and AVB, and indicated that NKX2.5 loss­of­function mutations are an uncommon cause of ASD and AVB in humans.

Calcium release near L-type calcium channels promotes beat-to-beat variability in ventricular myocytes from the chronic AV block dog.

Beat-to-beat variability of ventricular repolarization (BVR) has been proposed as a strong predictor of Torsades de Pointes (TdP). BVR is also observed at the myocyte level, and a number of studies have shown the importance of calcium handling in influencing this parameter. The chronic AV block (CAVB) dog is a model of TdP arrhythmia in cardiac hypertrophy, and myocytes from these animals show extensive remodeling, including of Ca(2+) handling. This remodeling process also leads to increased BVR. We aimed to determine the role that (local) Ca(2+) handling plays in BVR. In isolated LV myocytes an exponential relationship was observed between BVR magnitude and action potential duration (APD) at baseline. Inhibition of Ca(2+) release from sarcoplasmic reticulum (SR) with thapsigargin resulted in a reduction of [Ca(2+)]i, and of both BVR and APD. Increasing ICaL in the presence of thapsigargin restored APD but BVR remained low. In contrast, increasing ICaL with preserved Ca(2+) release increased both APD and BVR. Inhibition of Ca(2+) release with caffeine, as with thapsigargin, reduced BVR despite maintained APD. Simultaneous inhibition of Na(+)/Ca(2+) exchange and ICaL decreased APD and BVR to similar degrees, whilst increasing diastolic Ca(2+). Buffering of Ca(2+) transients with BAPTA reduced BVR for a given APD to a greater extent than buffering with EGTA, suggesting subsarcolemmal Ca(2+) transients modulated BVR to a larger extent than the cytosolic Ca(2+) transient. In conclusion, BVR in hypertrophied dog myocytes, at any APD, is strongly dependent on SR Ca(2+) release, which may act through modulation of the l-type Ca(2+) current in a subsarcolemmal microdomain.

RING finger protein RNF207, a novel regulator of cardiac excitation.

Two recent studies (Newton-Cheh, C. et al. (2009) Common variants at ten loci influence QT interval duration in the QTGEN Study. Nat. Genet. 41, 399-406 and Pfeufer, A. et al. (2009) Common variants at ten loci modulate the QT interval duration in the QTSCD Study. Nat. Genet. 41, 407-414) identified an association, with genome-wide significance, between a single nucleotide polymorphism within the gene encoding RING finger protein 207 (RNF207) and the QT interval. We sought to determine the role of RNF207 in cardiac electrophysiology. Morpholino knockdown of RNF207 in zebrafish embryos resulted in action potential duration prolongation, occasionally a 2:1 atrioventricular block, and slowing of conduction velocity. Conversely, neonatal rabbit cardiomyocytes infected with RNF207-expressing adenovirus exhibited shortened action potential duration. Using transfections of U-2 OS and HEK293 cells, Western blot analysis and immunocytochemistry data demonstrate that RNF207 and the human ether-a-go-go-related gene (HERG) potassium channel interact and colocalize. Furthermore, RNF207 overexpression significantly elevated total and membrane HERG protein and HERG-encoded current density by ∼30-50%, which was dependent on the intact N-terminal RING domain of RNF207. Finally, coexpression of RNF207 and HSP70 increased HERG expression compared with HSP70 alone. This effect was dependent on the C terminus of RNF207. Taken together, the evidence is strong that RNF207 is an important regulator of action potential duration, likely via effects on HERG trafficking and localization in a heat shock protein-dependent manner.

T-type calcium current contributes to escape automaticity and governs the occurrence of lethal arrhythmias after atrioventricular block in mice.

BACKGROUND: When complete atrioventricular block (AVB) occurs, infranodal escape rhythms are essential to prevent bradycardic death. The role of T-type Ca(2+) channels in pacemaking outside the sinus node is unknown. We investigated the role of T-type Ca(2+) channels in escape rhythms and bradycardia-related ventricular tachyarrhythmias after AVB in mice. METHODS AND RESULTS: Adult male mice lacking the main T-type Ca(2+) channel subunit Cav3.1 (Cav3.1(-/-)) and wild-type (WT) controls implanted with ECG telemetry devices underwent radiofrequency atrioventricular node ablation to produce AVB. Before ablation, Cav3.1(-/-) mice showed sinus bradycardia (mean±SEM; RR intervals, 148±3 versus 128±2 ms WT; P<0.001). Immediately after AVB, Cav3.1(-/-) mice had slower escape rhythms (RR intervals, 650±75 versus 402±26 ms in WT; P<0.01) but a preserved heart-rate response to isoproterenol. Over the next 24 hours, mortality was markedly greater in Cav3.1(-/-) mice (19/31; 61%) versus WT (8/26; 31%; P<0.05), and Torsades de Pointes occurred more frequently (73% Cav3.1(-/-) versus 35% WT; P<0.05). Escape rhythms improved in both groups during the next 4 weeks but remained significantly slower in Cav3.1(-/-). At 4 weeks after AVB, ventricular tachycardia was more frequent in Cav3.1(-/-) than in WT mice (746±116 versus 214±78 episodes/24 hours; P<0.01). Ventricular function remodeling was similar in Cav3.1(-/-) and WT, except for smaller post-AVB fractional-shortening increase in Cav3.1(-/-). Expression changes were seen post-AVB for a variety of genes; these tended to be greater in Cav3.1(-/-) mice, and overexpression of fetal and profibrotic genes occurred only in Cav3.1(-/-). CONCLUSIONS: This study suggests that T-type Ca(2+) channels play an important role in infranodal escape automaticity. Loss of T-type Ca(2+) channels worsens bradycardia-related mortality, increases bradycardia-associated adverse remodeling, and enhances the risk of malignant ventricular tachyarrhythmias complicating AVB.

Pharmacological characterization of the heartbeat in an extant vertebrate ancestor, the Pacific hagfish, Eptatretus stoutii.

Pharmacological ion-channel blockers were used to investigate the spontaneous heart rates in Pacific hagfish, Eptatretus stoutii. Zatebradine, a hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker, vastly reduced atrial and ventricular contraction rates in a similar concentration-dependent manner, indicating a major role for HCN in setting intrinsic heart rate. When voltage-gated Na(+) channels were blocked with tetrodotoxin (TTX), atrial contraction rate declined in a dose-dependent manner, but remained faster than ventricular rate even at very high TTX concentrations. This TTX resistance compared with other fish suggests an important role for a TTX-sensitive inactivation-resistant Na(+) current in atrioventricular conduction and chamber synchrony, and a lesser role in setting intrinsic heart rate. T and L-type calcium channel blockers, nickel and nifedipine respectively, also reduced atrial and ventricular contraction rates, nickel having a larger effect on the atrium. These novel results for hagfish are consistent with intrinsic atrial and ventricular rates being set mostly by HCN, with lesser contributions from other ion channels. We suggest that future electrophysiological studies will reveal that hagfishes, with their ancestral position in the evolution of the vertebrate-type chambered heart, share some but not all features of vertebrate intrinsic heart rate control.